Molecular epidemiological characteristics of Streptococcus pyogenes causing scarlet fever and angina in children

Objective To investigate the antimicrobial resistance, macrolide-resistance genes, virulence genes and emm types in Streptococcus pyogenes isolates. Methods A total of 247 oropharyngeal swab specimens were collected from pediatric outpatients (aged 2-11 years) who were clinically diagnosed as scarlet fever in Children′s Hospital of Fudan University from January to December, 2018. These specimens were timely sent to the Microbiology Laboratory for isolation and identification of Streptococcus pyogenes strains were isolate after culturing and identified with bacitracin susceptibility test. Moreover, the diameter of bacitracin inhibition zone was measured by vernier caliper. Their susceptibility to seven antibiotics, including erythromycin, clarithromycin, clindamycin, ampicillin, ceftriaxone, norfloxacin and chloramphenicol, were measured using KB method. Macrolide-resistance genes (mefA, ermA, ermB and Tn916 transposon) and virulence genes (speA, speB, speC, speG, speH, speI, speJ and speK) were detected by PCR. Amplification and sequencing of emm gene were conducted according to the protocol in the website of Centre for Disease Control and Prevention (CDC). Results A total of 86 strains of Streptococcus pyogenes were isolated from the 247 specimens. Their resistance rates to erythromycin, clarithromycin and clindamycin were 89.5%, 95.3% and 96.5%, respectively. However, these isolates showed high susceptibility to ampicillin (100.0%), ceftriaxone (100.0%), norfloxacin (90.7%) and chloramphenicol (95.3%). The positive rates of mefA, ermA, ermB and Tn916 genes were 20.9%, 24.4%, 98.8% and 97.7%. There was significant difference in the mefA-carrying rates between patients with scarlet fever and angina. The positive rates of virulence genes of speA, speB, speC, speG, speH, speI, speJ, speK, speL and speM were 8.1%, 100.0%, 95.3%, 100.0%, 80.2%, 90.7%, 10.5%, 100.0%, 5.8% and 5.8%. Seven emm types were identified and the predominant types were emm12.0 (75.6%), emm12.19 (9.3%) and emm1.0 (8.1%). The diameter of bacitracin inhibition zone was smaller in isolates of emm1.0 type than in emm12.0 type strains. The profile of virulence genes varied in the strains of different emm types. All types of strains carried speB, speG and speK genes. The isolates of emm1.0 type carried speA virulence gene, while speB, speC, speG, speH, speI and speK genes were more often identified in emm12.0 type isolates. Conclusions This study showed that emm types were associated with the profile of virulence genes and the diameter of bacitracin inhibition zone. It was recommended that the diameter of bacitracin inhibition zone should be measured in bacitracin inhibition susceptibility test apart from only observing the formation of inhibition zone. Key words: Streptococcus pyogenes; Antimicrobial resistance; Virulence gene; emm type; Bacitracin susceptibility test